Sheet-fed offset printing presses commonly utilize printing unit cylinders together with sheet transport rollers arranged between the printing units. These printing cylinders and transport rollers are typically driven by a continuous gear train with one or more drive motors. In order to achieve accurate printing, the gears of such a conventional gear train must be manufactured with very high precision.
Of course, manufacturing components at very low tolerances is expensive. This expense is substantial for printing presses, because of the numerous components, especially in the context of multiple printing cylinders driven by a continuous drive train.
Drive train elements conventionally requiring precision manufacture are circumferential and lateral registers and the adjusting devices required for correcting them. In order to correct a circumferential and/or lateral register, the plate cylinder is mounted such that it can be adjusted in the circumferential and/or lateral register direction (to the axial direction of the cylinder) with respect to the frame walls supporting it, and can also be adjusted with respect to the gear wheel supporting it. Also, in printing presses having helically toothed gears displacing the plate cylinder gear wheel relative to a blanket cylinder gear wheel driving it, simultaneously causing the plate cylinder gear wheel to rotate, additional correcting devices are also required.
Because of the foregoing reasons, developments have led to the individual driving of the cylinders and drums in sheet-fed offset printing presses by means of individual drives or combined in gear train groups. In this regard, Japanese patent publication JP-A-56-21860 discloses a drive for the cylinders of an offset printing unit, with the plate cylinder, the blanket cylinder and the impression cylinder respectively having individual motors. For the purpose of achieving angular synchronism, these individual drives follow commonly stipulated signals from an electronic control axle. Still, the individual cylinders of such an offset printing unit must be mutually driven with highly accurate angular synchronism. Accordingly, it is necessary to suppress interference or compensate for it. Deviations from synchronization occur due to different mass distribution among the printing unit cylinders. Specifically, this is due to periodically fluctuating loads as the cylinders roll against one another, and due to possible axial alignment errors between the motor and cylinder axis. Devices for compensating or for suppressing such interference are not provided.
German patent publication DE 4 137 979 Al discloses a drive for a printing press having multiple printing units in which the individual printing units or printing unit groups are mechanically decoupled from one another. Each printing unit or each printing unit group has a respective a drive motor and a device for determining rotational speed and/or angle of rotation. For achieving angular synchronism of these printing units or printing unit groups, angle controllers are provided which adjust a permissible deviation of the angle of rotation from a prescribed desired angular value, such that a deviation in the angle of rotation is minimal for the angle of rotation at which sheet transfer is performed. The aim of this device is particularly to avoid irregularities in sheet transfer from one printing unit to a neighboring printing unit or from one printing unit group to a neighboring printing unit group, something which otherwise leads to undesirable ghosting effects and color shifts which adversely affect print quality. Consequently, angular differences occurring between the desired angular value and the detected (actual) angular value are not corrected for each angular position, or at each instant by the angular position controller, but only, with the highest possible accuracy, at the instant of sheet transfer. Moreover, the previously known drive device is also intended to help to prevent the mechanical collision of the gripper paths in the case of sheet-guided cylinders.
The device of DE 4 137 979 Al places exceptional demands on the production and signaling accuracy of the sensor for determining the angle of rotation in conjunction with the rotating part on which the said sensor is mounted. Even the very slightest alignment error of the sensor or the rotor of this sensor with respect to the rotating part (i.e., the cylinder) coupled to it therefore causes a systematic and periodically repeating deviation of the actual angular value from the angular value supplied by the sensor. In this device, severe and periodically repeating fluctuations occur during each revolution of the cylinders in printing units of sheet-fed offset printing presses. These fluctuations are caused by gaps of the printing unit cylinders rolling against one another. Furthermore, in printing units of sheet-fed offset printing presses, compressible blankets are selected for use depending on various printing conditions, with the result that the drive or drives are subjected to differing drive torques. Additionally, the stickiness of ink printed in an offset printing unit produces large forces and increased drive torques, such that the drive power required is strongly affected by the printing form print area or by the print area proportion.
All of these factors must be considered in order to achieve the desired accuracy of synchronism among the cylinders. The influences are not taken into account or appropriately compensated in the device of DE 4 137 979 Al.
German Patent Publication DE 4 214 394 Al discloses a web-fed rotary press which has a number of individually driven cylinders and at least one separately driven folding unit. The individual drives of the cylinders and the drive controllers thereof are combined into press groups, all of which draw a positional reference from the folding unit. The press groups are managed by a higher-order control system (e.g. an electronic longitudinal shaft). Since this reference relates only to a drive system for a web-fed rotary press, the synchronism problems specific to sheet-fed offset printing presses are not addressed and are not solved. For example, this reference naturally does not address the problem of a so-called mechanical gripper collision which exists in sheet-fed arrangements but which does not exist in web-fed rotary presses. Additionally, the cylinders (e.g., plate, blanket and impression cylinders) in the printing unit of a web offset printing press have only very narrow cylinder gaps. Because the pronounced torque fluctuations occurring in sheet-fed offset printing presses do not occur when printing unit cylinders roll against one another, only very slight interference effects occur in a web-fed press.